Problem: On the first 3 tests of his language class, Ishaan got an average score of 86. What does he need on the next test to have an overall average of 89?
Answer: Let his score on the next test be $x$ The sum of all of his scores is then $3 \cdot 86 + x$ The same sum must also be equal to $4 \cdot 89$ Solve: $x = 4 \cdot 89 - 3 \cdot 86 = 98$.